Radio receiver with two tuners and a switch for verifying reception quality at an alternative frequency

ABSTRACT

A radio receiver having a circuit arrangement for checking the quality of reception at an alternative frequency to a received transmit frequency. The radio receiver has two tuners for converting the received signals to two different intermediate frequencies and a switch, which is preferably switched by a microprocessor, for optionally forwarding signals from signal processing branches that are each connected to one of the tuners. This reduces the cost of implementing the second receiver, at the same time reducing the barely noticeable impairment of the set program when checking the quality of reception at the alternate frequency. The latter is achieved by positioning the switch upstream from an analog-digital converter, operating at a sampling frequency, of an intermediate frequency stage. The different intermediate frequencies are selected so that they both meet the following condition: 
     
       
           f   0 =(n+R) f   a   
       
     
     where n is a natural number, and 0≦R≦1 applies. An intermediate frequency filter, a demodulator, possibly a decoder for additional signals, and a test unit for determining signal quality are connected to the analog-digital converter.

BACKGROUND INFORMATION

The present invention relates to a radio receiver having a circuitarrangement for checking the quality of reception at an alternativefrequency to a received transmit frequency, having two tuners forconverting the received signals to different intermediate frequenciesand a switch for optionally forwarding signals from signal processingbranches that are each connected to one of the tuners.

Modern radio receivers, particularly those used in multiple locations,as is preferably the case with car radios, are equipped with a circuitarrangement for checking the reception quality of a set station at analternative frequency of this station. The information about alternativefrequencies is generally derived from encoded information that istransmitted during a broadcast, for example using RDS (radio datasystem) signals. The reception quality at one or more alternativefrequencies is checked either regularly or when the reception quality ofthe selected station at the set frequency deteriorates.

Known methods include equipping the radio receiver with two separatereceivers for this purpose, with one receiver continuously selecting andchecking alternative frequencies in the background. For isolationpurposes and to avoid mutual disturbances, the two receivers havedifferent intermediate frequencies. A microprocessor compares thequality of the station received by the background receiver at thealternative frequency with the set program. This background check iscarried out without the user noticing any interruption in reception.However, this concept is relatively costly because both receivers mustbe fully equipped with tuners, IF filters, IF stages with demodulator,RDS decoders, signal quality test equipment, etc.

Another known method is to check the alternative frequencies using onlyone receiver. To do this, the tuner's PLL (phase-locked loop) brieflyswitches to an alternative frequency. Reception at the alternativefrequency is checked and a determination is made as to whether or notthe alternative frequency provides better reception than the frequencycurrently set. If reception at the alternative frequency is poorer, thereceiver switches back to the original frequency and possibly checksanother alternative frequency. Although this concept is more economical,it has the disadvantage that the check pauses and briefly, thoughnoticeably, interrupts the program to which the user is listening.

According to the related art, therefore, a problem arises in that eitherthe check of alternative frequencies can be carried out only at greaterexpense or, to reduce costs, the user must put up with a noticeableinterruption in the program to which he is listening.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to avoid the high costof using two separate and fully equipped receivers and also to at leastreduce interference with the received program.

According to the present invention, this object is achieved with a radioreceiver of the type mentioned in the preamble by positioning the switchupstream from an analog-digital converter, operating at a samplingfrequency (f_(a)), of an intermediate frequency stage; by selecting thedifferent intermediate frequencies so that they both meet the followingcondition:

f ₀=(n+R)f _(a)

where n is a natural number, and 0≦R≦1 applies; and an intermediatefrequency filter, a demodulator, possibly a decoder for additionalsignals, and a test unit for determining signal quality are connected tothe analog-digital converter.

According to the present invention, two tuners that convert the receivedsignal to different intermediate frequencies are used, like in theconcept with two receivers. However, both tuners are provided with acommon intermediate frequency stage having an intermediate frequencyfilter, a demodulator, a decoder for additional signals, and a test unitfor the signal quality, so that either the intermediate frequency of theone tuner or the intermediate frequency of the other tuner can bedirected to the intermediate frequency stage via the switch, which isdesigned, for example, as a multiplexer. This is possible because theanalog-digital converter operating at a sampling frequency (f_(a)) candigitize the different intermediate frequencies when both intermediatefrequencies meet the following condition:

f ₀=(n+R)f _(a)

as is generally known from German Patent Application No. 41 04 882. Thismakes it possible, according to the present invention, to process bothintermediate frequencies using the same intermediate frequency stagewithout any additional means.

To check the alternative frequencies, therefore, the switch, which ispreferably controlled by a microprocessor, interrupts processing of thesignals received by the set tuner, thus checking the alternativefrequency. Unlike the concept using one receiver, the interruption needsto be carried out only for the brief check period, while thecorresponding PLL must resonate to the new mixed frequency when a singletuner switches over, so that, when switching to an alternative frequencyand then back to the original frequency, two transient responses lastingjust milliseconds are added to the check time and form the overallinterrupt time. The interruption produced according to the presentinvention is therefore shorter by at least a factor of 2 so that theinterruption to the received program is barely noticeably. If thearrangement checks an alternative frequency that is known to have ausable reception quality, due to an earlier check, the interruption tothe received program is not noticeable at all.

If, however, there is the possibility that the reception at thealternative frequency is very poor, possibly causing a very noisy signalto be received, it is suitable to mute the low-frequency channel for thebrief check period to prevent any disturbing noise signals from passingthrough to the speaker.

Because the interruption during the check according to the presentinvention is so short that it does not noticeably disturb the receivedprogram, it may be suitable according to the present invention to createand store a displayable table of frequencies, i.e., stations, that canbe received at the reception location by checking a multiplicity offrequencies, making it possible to select a specific number of suitablestations, i.e., transmit frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows a block diagram of a radio receiver according to thepresent invention.

DETAILED DESCRIPTION

One embodiment of the present invention is illustrated schematically inthe FIGURE in the form of a block diagram. To a receiving antenna 1 areconnected two parallel tuner branches 2, 3, each of which includes apreliminary stage 4, 5 that amplifies the receive signal, a connectedtuner 6, 7, and a downstream filter 8, 9. Filters 8, 9 are notconventional intermediate frequency filters used to select the stationon the receiving band converted by tuner 6, 7, but rather serve only tolimit the band in order to cut off modulation-induced image frequenciesand similar phenomena. The outputs of both filters 8, 9 are connected totwo inputs of a switch 10 in the form of a multiplexer whose switchingstate is controlled by a microprocessor 11. Multiplexer 10 is normallyconnected so that the output signal of first tuner branch 2 is switchedthrough by multiplexer 10 and thus reaches an intermediate frequencystage 12, which is designed as a highly integrated circuit and includesan analog-digital converter for the intermediate frequency, anintermediate frequency filter for making the selection, a digitaldemodulator, an RDS decoder, and a test unit for the signal quality. Thesignal quality is checked in an essentially known manner by checking thefield strength, adjacent channel disturbances, multipath reception, anRDS error bit rate, etc.

A decline in the signal quality received via first tuner branch 2 isevaluated by microprocessor 11, causing microprocessor 11 to switchmultiplexer 10 so that a signal received by tuner 7, which waspreviously set to an alternative frequency by microprocessor 11, nowreaches intermediate frequency stage 12 at the intermediate frequency ofsecond tuner branch 3 via multiplexer 10.

The analog-digital converter of intermediate frequency stage 12 uses,for example, a sampling frequency of 14.25 MHz and is therefore suitablefor digitizing an intermediate frequency signal at a frequency of10.6875 MHz at the output of first tuner branch 2 as well as anintermediate frequency of 67.6875 MHz at the output of second tunerbranch 3, since these two intermediate frequencies meet the followingcondition:

f ₀=(n+R )f _(a)

where n=0 and R=0.75 for first tuner branch 2 and n=4 and R=0.75 forsecond tuner branch 3.

The reception quality at the alternative frequency is checked byintermediate frequency stage 12 and evaluated by microprocessor 11 sothat, upon detecting a better reception quality, multiplexer 10 remainsswitched, while a poorer reception quality causes multiplexer 10 toswitch back to the output signal of first tuner branch 2. In this case,this can be followed by a check of a further alternative frequency insecond tuner branch 3. In addition, the signal quality of alternativestation frequencies can be checked at specific time intervals (forexample, every five minutes), thereby continuously optimizing reception.

Microprocessor 11 generally receives the information about possiblealternative frequencies from the RDS signal that is decoded inintermediate frequency stage 12 and regularly includes a transmission ofinformation about alternative frequencies. However, it is also possibleto create and store a table of receivable frequencies and transmittedstation IDs by having microprocessor II sample numerous frequencies onthe radio frequency band, so that possible alternative frequencies canalso be derived from this table. However, the main function of the tableis to help the user operate the car radio by displaying, for example,the stations that can be received at the reception location on a screenand possibly allowing the user to select them directly. As shown in theFIGURE, both tuners 6, 7 can be set to the desired receiving frequenciesby microprocessor 11.

Because microprocessor 11 can set the PLLs included in both tuners 6, 7before multiplexer 10 switches, transient responses of the PLL of tuner6, 7, which is not currently switched through to intermediate frequencystage 12 by multiplexer 10, do not impair the audible signal.

Compared to the concept with one receiver, the cost of the circuitarrangement according to the present invention involves the provision ofa second preliminary stage, a second tuner, and a band limiting filterand is therefore comparatively low, since the much higher cost ofintermediate frequency stage 12 is not doubled. However, the relativelylow additional cost makes it possible to considerably reduce radioreception disturbances caused by checking alternative frequencies. Theuse of two different intermediate frequencies avoids disturbances causedby cross coupling and crosstalk, which would occur when using a sharedintermediate frequency.

What is claimed is:
 1. A radio receiver, comprising: a circuit forchecking a quality of reception at an alternative frequency to areceived transmit frequency; a first signal processing branch; a firsttuner connected to the first signal processing branch and for convertinga received signal to a first intermediate frequency; a second signalprocessing branch; a second tuner connected to the second signalprocessing branch and for converting the received signal to a secondintermediate frequency that is different than the first intermediatefrequency; an intermediate frequency stage including an analog-digitalconverter operating at a sampling frequency; an intermediate frequencyfilter connected to the analog-digital converter, a demodulatorconnected to the analog-digital converter; a test unit connected to theanalog-digital converter and for determining a signal quality; and aswitch positioned upstream from the analog-digital converter and foroptionally forwarding the received signal from the first signalprocessing branch and from the second signal processing branch, wherein:the first intermediate frequency and the second intermediate frequencymeet the condition f₀=(n+R)f_(a), f₀ corresponds to a respective one ofthe first intermediate frequency and the second intermediate frequency,f_(a) is the sampling frequency, n is a positive, whole number, and0≦R≦1 applies.
 2. The radio receiver according to claim 1, wherein: theintermediate frequency stage includes a decoder for additional signals.3. The radio receiver according to claim 1, further comprising: alow-frequency channel connected to the intermediate frequency stage andcapable of being muted according to a mute function while the switchswitches to the alternative frequency.
 4. The radio receiver accordingto claim 3, wherein: the mute function is capable of being deactivatedif an ability to receive the alternative frequency has already beenestablished.
 5. The radio receiver according to claim 1, wherein: adisplayable table of transmit frequencies that can be received at areception location can be created and stored by checking a plurality oftransmit frequencies.
 6. The radio receiver according to claim 1,wherein: the switch is capable of being switched by the test unit if thesignal quality of the received transmit frequency deteriorates.
 7. Theradio receiver according to claim 1, wherein: the switch is capable ofbeing switched to one of the first tuner and the second tuner receivingthe alternative frequency at fixed time intervals.